Power converter circuits may be used to convey power from a source, such as a battery, electrical power grid, etc. to a load, such as any device, apparatus, or component that runs on electricity, preferably with as little loss as possible. Generally, a power converter circuit provides an output voltage that has a different level than the input voltage. One type of power converter circuit is known as a switching or switched mode power supply. A switched mode power supply controls the flow of power from a power source to a load by controlling the “on” and “off” duty cycle of one or more transistor switches in order to regulate the DC output voltage across the output terminals of the power supply. The “on” and “off” duty cycle of the one or more transistor switches may be controlled in response to a pulse-width-modulated (PWM) gate drive signal provided by a switching regulator circuit, such that the “on” and “off” duty cycle of the one or more transistor switches is determined by relative pulse-widths of the PWM signal.
Switched mode power supplies have been implemented as an efficient mechanism for providing a regulated output, and are generally more power efficient than linear voltage regulators, which dissipate unwanted power as heat.
Some switched mode power supplies may use a transformer or an inductor as an energy transfer element and a capacitor as an energy storage element. A power transistor may be coupled to one side of the primary winding of a transformer, and may be turned on and off in response to the gate drive signal provided by the switching regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer may develop a rectified output voltage across a shunt output capacitor, which is typically an electrolytic capacitor, coupled across the secondary winding as a function of the energy transfer. The voltage across the output capacitor may provide the DC output voltage of the switching power supply.
In order to properly restart a power supply circuit, such as a power supply circuit that is used as a driver circuit for light emitting diodes (LEDs), it may be desirable to cause the energy stored in the output capacitor to drain out before restarting the power supply circuit. This will reduce the voltage on the output capacitor to a voltage that is below a point at which the circuit may be restarted properly upon re-application of the input voltage generated by the primary controller.
A conventional method of causing the output capacitor to drain is to provide a load resistor across the output capacitor. The load resistor will drain the output capacitor when the primary controller is shut down. However, the resistor will also dissipate power while the circuit is active. Furthermore, the resistor will need to have a sufficiently low resistance in order to effectively reduce the output capacitor voltage when the circuit is shut off. As a result, the resistor may undesirably dissipate a substantial amount of power.